Method and device for determining the nature of a formation at the head of drilling tool

ABSTRACT

The invention concerns a method for determining the nature of a formation ( 11 ) at the bottom ( 8 ) of a drilling well ( 4 ), the well ( 4 ) being equipped with a drill string ( 2 ) fitted with a drilling tool ( 14 ) and filled with a drilling fluid ( 18 ) circulating within the well ( 4 ) from inside to outside said drill string ( 2 ). According to the invention, the method includes the following steps:  
     differential measurement, between the outside and the inside of the drill string ( 2 ) in the vicinity of the drilling tool ( 14 ), of at least one physicochemical property of the drilling fluid ( 18 );  
     using the differential measurement to determine the nature of the formation ( 11 ) at the bottom ( 8 ) of the drilling well ( 14 ).  
     The invention also concerns a device ( 9 ) suitable for implementing such method.

TECHNICAL FIELD

[0001] The invention is related to the field of methods and devices for determining the nature of the formation at the head of a drilling tool. More specifically, the invention concerns methods and devices for determining the nature of a formation at the well bottom during drilling operations, so as to be able to adapt the drilling trajectory in relation to the results obtained.

PRIOR STATE OF THE ART

[0002] When drilling, it is preferable to be able to bring data on the formation located around the bottom of the drilling well back to the ground surface as fast as possible.

[0003] In effect, by determining as quickly as possible the nature of the formation at the level of the drilling tool positioned at the bottom end of a drill string, it is optionally possible to alter the direction of the drilling tool so that it moves into an area rich in hydrocarbons, or quite simply to prevent the tool from moving toward formations basically containing water.

[0004] In this technical field, several implementations have already been proposed.

[0005] In general, the drilling of a well is performed using a drilling tool caused to rotate with a drill string. The latter is made up of an assembly of hollow rods screwed one to another. A drilling fluid circulating within the well from inside to outside the drill string, also called drilling sludge, is used in particular for cooling and lubricating the drilling tool, and more particularly for removing formation debris cut away by the drilling tool.

[0006] Concerning the removal function of formation debris cut away by the drilling tool, a first type of method for determining the nature of a formation at the bottom of a well is known. Basically, this method consists of analysing the composition of the drilling fluid when it has carried out a complete cycle within the well, and it has reached ground level.

[0007] Since the drilling fluid is extracted continuously from the drilling well, automatic or manual collection of samples of this fluid is conducted on the surface. These samples are used to perform tests so as to determine one or more physicochemical properties of the constituent formation of the bottom of the well, by analysing, for example, some of the debris extracted from the fluid derived directly from the formation through which the drilling tool has cut.

[0008] Nevertheless, this type of method has a major drawback in connection with drilling depth, which is often considerable possibly reaching several kilometres.

[0009] When the drilling fluid is discharged from the drill string at the level of the drilling tool, it takes in debris derived from the formation through which the tool has drilled and then moves back towards ground level. In very deep drill holes, the fluid containing the debris able to provide information on the nature of the drilled formation sometimes only reaches ground level some hours after passing through the drilling tool. Consequently, the information does not reach ground level fast enough for its real time use in adapting the drilling trajectory.

[0010] A second type of method is also known for determining the nature of a formation at the bottom of a well.

[0011] In this type of method, measuring means are assembled on the drill string in the vicinity of the drilling tool. It is to be specified that the distance between the drilling tool and the measuring means is sufficient to prevent any damage to the measuring means since the latter is subjected to impacts and vibrations caused partly by the drilling tool. Due to the nonexistence of satisfactory mechanical protection, it is therefore recommended that the measuring means be installed a certain distance away from the drilling tool. By way of example, the measuring means are usually positioned approximately ten or twenty metres from the drilling tool, or at an even greater distance. The measuring means may be of any kind. Means are known which enable ‘Logging While Drilling’ techniques, which may, for example, measure the resistivity or the nuclear density of the formations drilled by the drilling tool.

[0012] In one such method, the data measured is generally transmitted to ground level using sound waves, which makes it possible to correct the direction of drilling, if necessary, almost instantaneously after the measurement has been taken.

[0013] However, one essential drawback remains at the time of implementing this type of method.

[0014] While it is true that the transmission of the physicochemical data measured is very fast, these data nevertheless concern physicochemical properties relating to the formations located at the level of the measuring means. On this account, the measurements made do not concern the formation forming the bottom of the well directly in contact with the drilling tool, but only concern the formation or formations located at some tens of metres, even at over one hundred metres above the bottom of the drilling well.

[0015] Consequently, when information on a physicochemical property of a formation reaches the ground surface, the drilling tool is already quite distant from that formation. Therefore, the drilling tool may be located in a formation having a totally different nature to that of the formation for which data is available. In such event, the contact of the drilling tool with a hostile formation would not be noted at ground level until some time afterwards, which may be as long as several hours.

[0016] Furthermore, this drawback is even more restrictive when hydrocarbon-rich formations are not very thick, for example, just a few metres deep, with the margin of error in the drilling trajectory then becoming more restricted.

[0017] The methods of the prior art using conventional drilling fluids therefore do not provide real time information on the physicochemical properties of the formation at the bottom of the well.

[0018] This situation not only causes substantial extra drilling costs through a non-optimised drilling trajectory, but also additional costs due to the drilling equipment used. Since a formation rich in hydrocarbons is generally in contact with an underlying formation mainly made up by water, if a drilling tool mistakenly reaches a water source, the hydrocarbons come into contact with and are mixed with the water. This then causes the formation of a hydrocarbon/water mixture making the material to be extracted from the drilling well considerably heavier. One possible consequence of this trajectory error is to make the well non-eruptive, requiring heavy pump-type means to pump out the hydrocarbon/water mixture from the well, together with costly means for separating the water from the hydrocarbons and re-injecting the water into the ground.

[0019] Also, it is to be pointed out that the lack of real-time control over the drilling trajectory may also represent a major explosion risk, should the drilling tool perforate a cavity containing gas under heavy pressure.

DESCRIPTION OF THE INVENTION

[0020] The purpose of the invention is therefore to present a method and a device for determining the nature of a formation at the head of a drilling tool, thereby overcoming, at least in part, the above-described drawbacks relating to the implementations of the prior art.

[0021] The present invention also sets out to propose a method and device enabling almost real-time delivery of information on the formation that makes up the bottom of the drilling well.

[0022] To achieve these purposes, the subject of the invention is firstly a method for determining the nature of a formation at the bottom of a drilling well, the well being equipped with a drill string fitted with a drilling tool and filled with a drilling fluid circulating within the well from inside to outside said drill string. According to the invention, the method comprises the following steps:

[0023] differential measurement, between the outside and the inside of the drill string in the vicinity of the drilling tool, of at least one physicochemical property of the drilling fluid;

[0024] using the differential measurement to determine the nature of the formation at the bottom of the drilling well.

[0025] Advantageously, the method according to the invention, allows very fast determination of the nature of the formation that makes up the bottom of the drilling well using a single measurement of an appropriate physicochemical property or properties of the drilling fluid.

[0026] In effect, the time required for the drilling fluid to circulate between the formation at the bottom of the well, where it takes in debris, and the measuring means positioned in the vicinity of the drilling tool, remains short and does not exceed a few minutes.

[0027] Thus, information is available with which to determine the nature of the formation at the bottom of the well in distinctly quicker manner than with the methods of the prior art.

[0028] In addition, with the differential measurement obtained it is possible to overcome variations in measurement due to changes in measuring conditions caused, for example, by a temperature increase in the drilling fluid as and when the drilling well increases in depth.

[0029] According to a first preferred embodiment, the determination of the nature of the formation at the bottom of the drilling well consists of qualitatively determining at least one physicochemical property of the formation at the bottom of the well. In this way, a simple comparison between the physicochemical properties of the drilling fluid and those of the formation at the bottom of the well, lead to easy determination of the nature of this formation.

[0030] Also in a second preferred embodiment of the invention, provision may also be made so that determination of the nature of the formation at the bottom of the drilling well consists of a quantitative determination of at least one physicochemical property of the formation at the bottom of the well. In this case, in addition to qualitative knowledge of at least one physicochemical property of the formation at the bottom of the well, it is also possible to deduce the value of these physicochemical properties relative to the formation.

[0031] According to a third preferred embodiment of the invention, the determination of the nature of the formation may consist of comparing at least one measured physicochemical property of the drilling fluid with pre-set values. Advantageously, these pre-set values are able to provide direct information on the nature of the formations encountered by the drilling tool.

[0032] It is to be specified that the method may include a data-transmission step for the transmission of information from the bottom to the top surface of the well, and this may be conducted continuously when drilling.

[0033] A further subject of the invention is a device for determining the nature of a formation at the bottom of a drilling well, the well being equipped with a drill string fitted with a drilling tool and filled with a drilling fluid circulating within the well from inside to outside said drill string, said device comprising primary measuring means and secondary measuring means mounted on the drill string in the vicinity of the drilling tool. According to the invention, the primary measuring means and the secondary measuring means are suitable for measuring at least one physicochemical property of the drilling fluid respectively located outside and inside said drill string, so as to obtain a differential measurement of each measured physicochemical property, at least one physicochemical property measured enabling determination of the nature of the formation at the bottom of the drilling well.

[0034] Further, the primary measuring means are able to measure at least one of the physicochemical properties of the drilling fluid chosen from among the group comprising impedance, pH, nuclear density, electric voltage and chemical tracers which react with the fluid contained in the formation.

[0035] Other characteristics and advantages of the invention will become apparent in the non-restrictive description given below.

BRIEF DESCRIPTION OF THE DRAWINGS

[0036] The description is made with reference to the single FIGURE showing a schematic view of a drilling assembly comprising a device for determining the nature of a formation at the bottom of a drilling well according to one preferred embodiment of the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0037] With reference to the single figure, a drilling assembly 1 can be seen which comprises in particular a drill string 2 carrying a drilling tool 14 at its lower end. The drilling assembly 1 is located inside a drilling well 4, being delimited upwards by the ground level 6 and downwards by the well bottom 8 with which the drilling tool 14 is in contact.

[0038] The drilling assembly 1 also comprises a device 9 for determining the nature of a formation 11 at the bottom of a drilling well 8, corresponding to the formation 11 located at the head of the drilling tool 14.

[0039] In this embodiment, the device 9 includes several parts including a data transmitter 10 and a measuring unit 12, these parts being mounted respectively one on top of one another on the drill string 2, in the direction leading from the bottom of the drilling well 8 to the ground surface level 6, at the level of a lower part 2 a of this drill string 2. The measuring unit 12 comprises primary measuring means 26, secondary measuring means 28 and tertiary measuring means 30.

[0040] The device 9 also includes a data receiver 16 assembled in an upper part 2 b of the drill string 2, and cooperating with the data transmitter 10 in the lower part 2 a of the drill string 2.

[0041] The drilling tool 14 is located in the vicinity of the primary measuring means 26, and preferably in the vicinity of each of measuring units 26, 28, 30. It is to be specified that the average distance 15 between the part of the tool 14 in contact with the well bottom 8 and the measuring means 26, 28, 30 is in the order of a few dozen metres. This distance 15 could evidently be shorter, but a value of approximately one hundred metres seems fully appropriate for protecting measuring means 26, 28, 30 from impacts and vibrations caused by drilling tool 14.

[0042] The drilling assembly 1 also includes a drilling fluid added continuously from the ground surface level 6 to the inside of a flexible pipe 20, via a reservoir (not shown) of drilling fluid 18.

[0043] It is to be noted that the drilling fluid 18 fills a cylindrical space 22 inside the drill string 2 and also a ring-shaped space 24 located between the wall of well 4 and the drill string 2.

[0044] When boring a drill well 4, the drilling fluid 18 enters the flexible pipe 20 under pressure as schematically outlined by arrow A in the single figure. The fluid 18 therefore enters space 22 inside the drill string 2, then follows the direction of arrow B towards the well bottom 8.

[0045] After passing through the measuring unit 12, the fluid is discharged from the drill string 2 via the drilling tool 14, as shown by arrow C, then it moves up into ring-shaped space 24 located between the drilling well 4 and the drill string 2.

[0046] To conduct a drilling operation, the drilling tool 14 is driven with a rotating movement produced by means (not shown) located at ground surface level 6, or by the drilling fluid 18. In addition, the drilling tool 14 is also subjected to a translational movement in the direction leading from ground surface level 6 to the well bottom 8, which enables it to cross through and fragment a formation 11 formed by well bottom 8.

[0047] Formation cuttings (not shown) are created at well bottom 8, and they are extracted from the well 4 moving up with the drilling fluid 18 into the ring-shaped space 24, as is shown by arrow D in the single figure.

[0048] The drilling fluid 18, containing cuttings, is then expelled from the drilling well 4 via a rigid pipe 34 located above the ground surface level 6.

[0049] According to the invention, the primary measuring means 26 and the secondary measuring means 28 are able to measure at least one physicochemical property of the drilling fluid 18 respectively positioned outside and inside the drill string 2, so as to obtain a differential measurement of each measured physicochemical property, at least one measured physicochemical property able to be used to determine the nature of the formation 11 of the bottom 8 of the drilling well 4.

[0050] In this way, a differential measurement is obtained of the physicochemical property of the drilling fluid 18, which makes it possible to cope with any variations in the physicochemical property which are not due to a change in the nature of formation 11, but which may possibly derive in particular from a change in depth of the drilling well 4.

[0051] The measured physicochemical property or properties of fluid 18 circulating in the ring-shaped space 24, may in particular be impedance, pH, nuclear density through measured absorption of gamma-ray photons, or even electric voltage.

[0052] The measured physicochemical properties of the drilling fluid 18 may evidently be of any kind, provided they give information on the nature of the formation 11 forming well bottom 8. Persons skilled in the art are able to determine the appropriate physicochemical properties of the fluid 18 to be measured, so that the cutting debris mixed with the fluid 18 and derived from the drilled formations 11 may possibly cause the chosen physicochemical property or properties to vary in relation to the nature of formation 11. It is to be noted by way of example that it is possible to group formations into three types of different natures, namely formations which do not contain any hydrocarbons, formations containing hydrocarbons and formations not containing any hydrocarbons and essentially formed of water.

[0053] Determination of the nature of the formation 11 at well bottom 8 may be performed in different ways. Several preferred embodiments of the invention are described below, in which the primary measuring means 26 and the secondary measuring means 28 only measure one single physicochemical property of the drilling fluid 18. These embodiments may evidently also apply to cases when a plurality of physicochemical properties are measured.

[0054] Firstly, it is to be pointed out that the measured physicochemical property is transmitted from the primary 26 and secondary 28 measuring means towards the ground surface level 6, via the data transmitter 10 directly linked to measuring means 26, 28, and via the data receiver 16 cooperating with the transmitter 10. In general, data derived from the transmitter 10 is transmitted by sound waves circulating in the drilling fluid 18 within the cylindrical space 22 located inside the drill string 2. Data on the measured physicochemical property of the fluid 18 is then analysed at ground surface level 6, preferably in continuous manner during the conducting of a drilling operation.

[0055] According to a first preferred embodiment of the invention, the determination of the nature of formation 11 at well bottom 8 consists of a qualitative determination of a physicochemical property of the formation 11 at well bottom 8. In other words, the measured physicochemical property of the drilling fluid 18 is such that its variation over time can be used to deduce directly the variation of a physicochemical property relating to formation 11 at well bottom 8, the deduced variation in this physicochemical property being able to provide direct information on a change in the nature of the drilled formation 11. Indeed, the variation in the physicochemical property of formation 11, such as resistivity for example, allows detection of changes in the nature of the drilled formations by means of a simple analysis, such as the change when tool 14 crosses from a formation rich in hydrocarbons to a formation essentially formed of water.

[0056] Also, when there is a well-known correlation between the physicochemical property of the drilling fluid 18 and a physicochemical property relating to the formation 11, a simple variation in the physicochemical property of the drilling fluid 18 can directly provide information on a change in the nature of the formation 11. Consequently very swift deducing can be made of the nature of the formation 11 located at the head of the drilling tool 14.

[0057] According to a second preferred embodiment of the invention, in addition to the qualitative determination of a physicochemical property of formation 11 at well bottom 8, it is also possible to make provision for this determination to be carried out in quantitative manner.

[0058] To do so, global use is made of the same technique as previously with the difference that each measured value of the physicochemical property of fluid 18 is caused to correspond to a value of the physicochemical property of the drilled formation 11. It is thus possible to determine more accurately the composition of formation 11 cut by the drilling tool 14, in particular when the deduced physicochemical property concerns resistivity or the density of formation 11.

[0059] By way of example, provision may be made for the tertiary measuring means 30 to measure a physicochemical property of a formation 33 located at the level of measuring means 12, and more precisely at the level of tertiary measuring means 30. Nonetheless, due to the distance 15 existing between the tertiary measuring means 30 and the formation 11 at well bottom 8, this value will only reach the surface a few hours after the arrival at ground surface level 6 of the value of the physicochemical property of the drilling fluid 18. Despite this time lag, the tertiary measuring means 30 are adapted so as to conduct logging to indicate the rotation and penetration speeds of tool 14 and the drilling trajectory. In addition, these tertiary means 30 can also provide information on the physicochemical properties of formation 33, such as natural radioactivity, resistivity, or even density.

[0060] In order to prevent the time lag in the measurements mentioned above, it is possible to use other factors to determine the value of the physicochemical property of formation 11 at well bottom 8. Prior to the drilling operations, tests may be performed to determine the correlation existing between the values of the physicochemical property of fluid 18 and the values of the physicochemical property of formation 11 at well bottom 8. It is therefore not necessary to wait for the measurements arriving from the tertiary measuring means 30 in order to determine the value of the physicochemical property of formation 11 cut by tool 14, and faster determination can therefore be made of the nature of this formation 11.

[0061] In a third preferred embodiment of the invention, the determination of the nature of formation 11 may consist of simply comparing the values obtained for the physicochemical property of the fluid 18 measured by the primary 26 and secondary 28 measuring means, with pre-set values for this same physicochemical property. In this way, it is no longer obligatory to conduct an intermediate step for determining a physicochemical property of formation 11. The pre-set values of a judiciously chosen physicochemical property for fluid 18 may be set out in graph form, so that the measurement made can simply be read off to obtain direct information on the nature of formation 11.

[0062] In the method for determining the nature of formation 11 at well bottom 8, the primary 26 and secondary measuring means are positioned in the vicinity of the drilling tool and preferably a few dozen metres from that part of tool 14 in contact with well bottom 8. Consequently, since the average rise speed of drilling fluid 18 into the ring-shaped space 24 is approximately a few metres per second, the physicochemical property of the fluid 18 containing cutting debris from formation 11 can then be measured less than one minute after the tool 14 has cut through this formation 11. With the information on this physicochemical property being immediately transmitted to ground surface level 6 via transmitter 10 and receiver 16, adaptation of the drilling trajectory, by orienting tool 14, can thus be performed very quickly. In this case, between the time when the drilling tool 14 has cut through a formation and the time when it is possible to determine the nature of this formation, the drilling tool 14 will only have advanced a few centimetres into well bottom 8.

[0063] With such a method, almost total optimisation of the drilling trajectory can therefore be considered.

[0064] Various modifications may evidently be made by persons skilled in the art to the method and device 9 just described solely for illustrative purposes and non-restrictive. 

1. Method for determining the nature of a formation (11) at the bottom (8) of a drilling well (4), the well (4) being equipped with a drill string (2) fitted with a drilling tool (14) and filled with a drilling fluid (18) circulating within the well (4) from inside to outside said drill string (2), characterised in that it comprises the following steps: differential measurement between the outside and the inside of the drill string (2), in the vicinity of the drilling tool (14), of at least one physicochemical property of the drilling fluid (18); using the differential measurement to determine the nature of the formation (11) at the bottom (8) of the drilling well (4).
 2. Method according to claim 1, characterised in that the determination of the nature of the formation (11) at the bottom (8) of drilling well (4) consists of a qualitative determination of at least one physicochemical property of the formation (11) at the bottom (8) of drilling well (4).
 3. Method according to claim 1 or claim 2, characterised in that the determination of the nature of formation (11) at the bottom (8) of the drilling well (4) consists of a quantitative determination of at least one physicochemical property of the formation (11) at the bottom (8) of the drilling well (4).
 4. Method according to claim 1, characterised in that the determination of the nature of formation (11) at the bottom (8) of the drilling well (4)is made by comparison with pre-set values.
 5. Method according to any of the preceding claims, characterised in that it comprises a data transmission step, from the bottom (8) of the drilling well (4) towards a ground surface level (6).
 6. Method according to any of the preceding claims, characterised in that measurement is made of at least one of the physicochemical properties of drilling fluid (18) chosen from among the group made up of electrical impedance, pH, nuclear density and electric voltage.
 7. Method according to any of the preceding claims, characterised in that it is implemented continuously during a drilling operation.
 8. Device (9) for determining the nature of a formation (11) at the bottom (8) of a drilling well (4), the well (4) being equipped with a drill string (2) fitted with a drilling tool and filled with a drilling fluid (18) circulating within the well (4) from inside to outside said drill string (2), the said device comprising primary (26) measuring means and secondary measuring means (28) mounted on the drill string (2) in the vicinity of the drilling tool (14), characterised in that the primary measuring means (26) and the secondary measuring means are suitable for measuring at least one physicochemical property of the drilling fluid (18) respectively located inside and outside said drill string (2), so as to obtain a differential measurement of each measured physicochemical property, at least one measured physicochemical property enabling determination of the nature of formation (11) at the bottom (8) of the drilling well (4).
 9. Device (9) according to claim 8, characterised in that the device (9) also comprises tertiary measuring means (30) mounted on the drill string (2) in the vicinity of the drilling tool (14) and suitable for measuring at least one physicochemical property of a formation (33) located at the level of the tertiary measuring means (30).
 10. Device (9) according to claim 8 or claim 9, characterised in that the device (9) comprises a data transmitter (10) assembled on the drill string (2) in the vicinity of the measuring means (26, 28, 30), and a data receiver (16) positioned outside the drilling well (4).
 11. Device (9) according to any of claims 8 to 10, characterised in that the primary (26) and secondary (28) measuring means are able to measure at least one of the physicochemical properties of the drilling fluid (18) chosen from the group comprising impedance, pH, nuclear density and electric voltage. 